Molecular approaches to study bud dormancy in Populus

Most physiological research on tree growth and dormancy has focused on the environmental control of the growth periodicity. Much less has been contributed to the understanding of the endogenous factors responsible for the integration of environmental signals into the organogenesis of the bud. This is partially due to a lack of early indicators that allow the monitoring of the effects of subtle changes in the environment on the growth activity of the bud. To reveal these differences in single buds we have analyzed their cell cycle activity individually. Populus has been transformed with chimeric beta-glucuronidase (gus) genes under the control of cell cycle-specific promoters (Arabidopsis cdc2 and cyc1). After having confirmed a similar expression pattern as in Arabidopsis, we have analyzed the expression levels of both chimeric genes in individual buds of the same tree, and under different conditions known to promote or retard growth in the buds. The expression levels of both chimeric genes were found to act as early and sensitive indicators of the growth activity in the buds. Second, common cytological and physiological characteristics of bud and seed dormancy might suggest that they are based on similar processes at the molecular level. Promoter-gus fusions, with the promoters conferring expression in the seed, were assayed for gus expression in the apical buds of Arabidopsis under growth-inhibiting conditions. Pabi3-gus, so far believed to be seed specific, was demonstrated to be expressed in arrested Arabidopsis apices and abi3-homologous transcripts were detected in poplar buds.